Error setting rowname if rowname currently NULL

fred<-matrix(,4,2)
fred

rownames(fred)

rownames(fred) <- c("Apple",'','','')
fred

Hi,

My first post here and new to R so please bear with me (long time programmer
though, helping a friend with some scripts).

I've noticed a behaviour when using rownames() that I think is odd,
wondering if I'm doing something wrong.

To illustrate, say I create a very simple matrix (called fred):

fred<-matrix(,4,2)

It looks like this:

? ? [,1] [,2]
[1,] ? NA ? NA
[2,] ? NA ? NA
[3,] ? NA ? NA
[4,] ? NA ? NA

If I now try and set a row name for one of the rows (say the first row) to
"APPLE", by doing this:

rownames(fred)[1] <- "APPLE"

I get an error:

Error in dimnames(x) <- dn :
?length of 'dimnames' [1] not equal to array extent

However, I found that if I first set all the rownames to anything at all, by
using say:

rownames(fred) <- c(1:4)

Which gives me:

?[,1] [,2]
1 ? NA ? NA
2 ? NA ? NA
3 ? NA ? NA
4 ? NA ? NA

Then my desired command works, and thus:

rownames(fred)[1] <- "APPLE"

Gives me what I want:

? ? ?[,1] [,2]
APPLE ? NA ? NA
2 ? ? ? NA ? NA
3 ? ? ? NA ? NA
4 ? ? ? NA ? NA

So, what this says to me is that to set the row names INDIVIDUALLY, they
first need to be set to something (anything!).

For what I am doing, I need to set the row names one at a time, as I iterate
through a loop. So I found that to do this I first had to set the rownames
to some dummy values as above. Then it works fine. But this seems a little
kludgy and unnecessary to me, and I am wondering what I am doing wrong. Have
just started in R so fumbling my way through somewhat.

Any suggestions would be appreciated, thanks!


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